Add a 'Difference Chart' to the Chart API.

This can be done in JFreeChart using the XYDifferenceRenderer, here is a link to an example:
http://www.jfree.org/jfreechart/api/javadoc/org/jfree/chart/renderer/xy/XYDifferenceRenderer.html

This is also used in JIRA for the "Issues: 30 Day Summary" to highlight the difference between issues created and issues resolved.

Note that if the two lines cross, the fill color between them should change.